Taurine can be referred to as 2-aminoethanesulfonic acid and is one of the amino sulfonic acids found in the tissues of many animals. Taurine is an extremely useful compound with beneficial pharmacological effects, such as detoxification, fatigue-relief, and nourishing and tonifying effects. As a result, taurine finds wide applications as an essential ingredient for human and animal nutrition.
Taurine is currently produced in an amount of over 50,000 tons per year from either ethylene oxide or monoethanolamine. At the present time, most taurine is produced from ethylene oxide, following a three-step process: (1) the addition reaction of ethylene oxide with sodium bisulfite to yield sodium isethionate; (2) the ammonolysis of sodium isethionate to yield sodium taurinate; (3) the neutralization with an acid, i.e., hydrochloric acid and, preferably, sulfuric acid, to generate taurine and inorganic salts.
Although the ethylene oxide process is well established and widely practiced in commercial production, the overall yield is not very high, less than 80%. Moreover, the process generates a large waste stream that is increasingly difficult to dispose of.
The first stage of the ethylene oxide process, the addition reaction of ethylene oxide with sodium bisulfite, is known to yield sodium isethionate in high yield, practically quantitative, as disclosed in U.S. Pat. No. 2,820,818 under described conditions.
Therefore, the problems encountered in the production of taurine from the ethylene oxide process arise from the ammonolysis of sodium isethionate and from the separation of taurine from sodium sulfate
U.S. Pat. No. 1,932,907 discloses that sodium taurinate is obtained in a yield of 80%, when sodium isethionate undergoes ammonolysis reaction in a molar ratio of 1:6.8 for 2 hours at 240 to 250° C. U.S. Pat. No. 1,999,614 describes the use of catalysts, i.e., sodium sulfate, sodium sulfite, and sodium carbonate, in the ammonolysis reaction. A mixture of sodium taurinate and sodium ditaurinate is obtained in a yield as high as 97%. However, the percentage for sodium taurinate and sodium ditaurinate in the mixture is not specified.
DD219023 describes detailed results on the product distribution of the ammonolysis reaction of sodium isethionate. When sodium isethionate undergoes the ammonolysis reaction with 25% aqueous ammonia in a molar ratio of 1:9 at about 280° C. for 45 minutes in the presence of sodium sulfate and sodium hydroxide as catalyst, the reaction products comprise 71% of sodium taurinate and 29% of sodium di- and tritaurinate.
WO01/77071 is directed to a process for the preparation of ditaurine by heating an aqueous solution of sodium taurinate at a temperature of 210° C. in the presence of a reaction medium. A mixture of sodium taurinate and sodium ditaurinate is obtained.
It is therefore concluded from the foregoing references that the ammonolysis of sodium isethionate invariably yields a mixture of sodium taurinate, sodium ditaurinate, and sodium tritaurinate. The percentage yield of sodium taurinate has not been more than 80%.
In order to obtain taurine from sodium taurinate, U.S. Pat. No. 2,693,488 discloses a method of using ion exchange resins involving a strongly acid ion exchange resin in hydrogen form, and then an anion exchange resin in basic form. This process is complicated and requires the use of a large quantity of acid and base to regenerate the ion exchange resins in each production cycle.
On the other hand, CN101508657, CN101508658, CN101508659, and CN101486669 describe a method of using sulfuric acid to neutralize sodium taurinate to obtain a solution of taurine and sodium sulfate. Crude taurine is easily obtained by filtration from a crystalline suspension of taurine after cooling. However, the waste mother liquor still contains taurine, sodium sulfate, and other unspecified organic impurities, which are identified as a mixture of sodium ditaurinate and sodium tritaurinate.
U.S. Pat. No. 9,428,450 and U.S. Pat. No. 9,428,451 overcome some of the problems in the known ethylene oxide process by converting the byproducts of the ammonolysis reaction of alkali isethionate, alkali ditaurinate and alkali tritaurinate, into alkali taurinate. The overall yield of the cyclic process for producing taurine from sodium isethionate is increased to from 85% to nearly quantitative.
U.S. Pat. No. 8,609,890 discloses a process of using isethionic acid or sulfur dioxide to neutralize alkali taurinate to producing taurine and to regenerate alkali isethionate. U.S. Pat. No. 9,108,907 further discloses a process of using isethionic acid prepared from ethanol to neutralize alkali taurinate to regenerate alkali isethionate. The aim is to reduce or eliminate the use of sulfuric acid as an acid agent in the production of taurine.
U.S. Pat. No. 9,061,976 discloses an integrated production scheme by using sulfur dioxide as an acid and by converting the byproducts of the ammonolysis reaction, alkali ditaurinate and alkali tritaurinate, to alkali taurinate. The overall production yield is increased to greater than 90% and alkali sulfate is eliminated from the production process. One drawback of this process is the use of gaseous sulfur dioxide, which imparts a slight smell on the product. Another significant drawback is that the taurine product from this process may contain trace amount of alkali sulfite which could be an allergen for certain people.
Copending U.S. Ser. No. 15/238,621 discloses a cyclic process for producing taurine from isethionic acid in a high overall yield of greater than 90% to nearly quantitative, while generating no inorganic salt as byproducts. However, the starting material, isethionic acid, is difficult to obtain commercially and is produced by a costly process of bipolar membrane electrodialysis of alkali isethionate.
CN 101717353A describes a process of preparing taurine by (1) reacting ethylene oxide with ammonium sulfite to yield ammonium isethionate and ammonia; (2) ammonolysis of the obtained product to ammonium taurinate; (3) acidifying with sulfuric acid to afford taurine. However, repeated attempts fail to produce any taurine under disclosed conditions.
It is an object of the present invention to overcome the disadvantage of the known processes for the production of taurine and to provide, in addition, advantages, which will become apparent from the following description.
It is another object of the present invention to disclose a process for the production of taurine from ammonium isethionate in a high overall yield (i.e., greater than 90% to nearly quantitative) without generating any inorganic salt as byproduct.
The starting material, ammonium isethionate, can be readily and economically produced by reacting ethylene oxide with ammonium bisulfite according to prior art, e.g., U.S. Pat. No. 5,646,320 and U.S. Pat. No. 5,739,365.
According to the process of the present invention, a solution of alkali isethionate or regenerated alkali isethionate, alkali ditaurinate, and alkali tritaurinate is mixed with an excess ammonia and is subjected continuously to the ammonolysis reaction to form a mixture of alkali taurinate, alkali ditaurinate, and alkali tritaurinate, in the presence of one or more catalysts. After ammonium isethionate is added to the ammonolysis solution, excess ammonia is removed to obtain a crystalline suspension of taurine in a solution of alkali isethionate, alkali ditaurinate, and alkali tritaurinate. Upon the solid-liquid separation of taurine, the mother liquor is directly recycled to the ammonolysis step.
The advantage of using ammonium isethionate as a starting material becomes apparent in that no isolation of alkali salt as a byproduct is necessary after the separation of crystalline taurine from the mother liquor containing alkali isethionate, alkali ditaurinate, and alkali tritaurinate.